The present invention relates to a controller and control method for an internal combustion engine that is provided with an intake path and an exhaust path, a throttle valve for opening and closing the intake path, an EGR path for recirculating exhaust gas from the exhaust path to an intake manifold which is a part of the intake path at the downstream side of the throttle valve, and an EGR valve for opening and closing the EGR path.
In order to appropriately control an internal combustion engine, it is important to accurately calculate the amount of air taken into a cylinder and to accurately control the fuel supply amount and the ignition timing in accordance with the cylinder intake air amount. Ignition timing control needs to change an ignition timing to the ignition timing (MBT: Minimum Spark Advance for Best Torque) at which the output torque becomes maximal or the like, in accordance with not only the rotation speed of the internal combustion engine and the cylinder intake air amount but also other factors such as the coolant temperature of the internal combustion engine, whether or not a knock has occurred, the fuel property, and the EGR (Exhaust Gas Recirculation) rate.
Meanwhile, with regard to the EGR, there are two methods, i.e., a method (referred to as an external EGR, hereinafter) in which an EGR valve is provided in an EGR path for recirculating exhaust gas from the exhaust path to the intake manifold and the amount of exhaust gas to be recirculated to the intake manifold is controlled based on the opening degree of the EGR valve and a method (referred to as an internal EGR, hereinafter) in which a variable valve timing mechanism, which makes the opening/closing timings of one of or both of an intake valve and an exhaust valve variable, is provided and an overlap period, during which the intake valve and the exhaust valve are concurrently opened, is changed so that the amount of exhaust gas remaining in the cylinder is controlled. In recent years, in order to reduce the fuel cost and raise the output, the number of internal combustion engines provided with both the external EGR mechanism and the internal EGR mechanism has been increasing. In the present invention, an EGR and an EGR rate, when simply described in this manner, denote an external EGR and an external EGR rate, respectively.
In recent years, an internal combustion engine has been controlled by utilizing, as an index, the output torque of the internal combustion engine. Because the thermal efficiency changes depending on the cylinder intake air amount and the EGR rate, it is required to estimate the output torque based on the cylinder intake air amount and the EGR rate. Accordingly, either in order to control the ignition timing or in order to estimate the output torque, it is required to accurately estimate the EGR rate.
As a technology for estimating an EGR rate, for example, the technology disclosed in Japanese Patent Application Publication No. H07-279774 (JP H07-279774 A) has already been known. In the technology of the JP H07-279774 A, the flow rate QACT of the recirculation exhaust gas according to the opening area of the EGR valve is estimated using the flow characteristic of the EGR valve, and the flow rate QCMD of the recirculation exhaust gas according to the command value of the opening area of the EGR valve is estimated using the flow characteristic of the EGR valve. Based on the flow rates QACT and QCMD of recirculation exhaust gas, it configures so that an EGR rate may be estimated.